Directional couplers comprise two waveguides formed in electro-optic material such as lithium niobate, lithium tantalate or gallium arsenide for example, disposed in close proximity over a coupling length selected to enable evanescent coupling of 100% of the transmitted optical power from one waveguide to the other. An electric field created by a voltage applied through one or both waveguides alters the propagation constant of the waveguide due to the electro-optic effect of the substrate, resulting in a change in the percentage of coupling. Directional couplers are useful in optical switches, modulators and variable attenuators.
Directional couplers have been difficult to manufacture successfully for switching and related applications, in part because the manufacturing tolerances to achieve the proper coupling length and precise symmetry required have been commercially challenging, and in part because the wavelength range of the coupler has been narrower than necessary for the application. If full coupling does not occur from one waveguide to the other because the coupling length is not correct, excess optical loss is introduced and/or full extinction cannot be achieved, thus making the coupler unsuitable for many applications.
In response, asymmetric directional couplers have been developed in which the two waveguides have different propagation constants. The asymmetric directional coupler has been demonstrated to provide coupling over a broader wavelength band, for example in the article “Design and Fabrication of Broad-Band Silica-Based Optical Waveguide Couplers with Asymmetric Structure,” by Akihiro Takagi et al. in IEEE Journal of Quantum Electronics, Vol. 28, No. 4, April 1992. Also reduced wavelength sensitivity has been demonstrated in an asymmetric coupler in electro-optic material disclosed in U.S. Pat. No. 6,842,569 by Suwat Thaniyavarn issued to Eospace Inc. Jan. 11, 2005. In this case a pair of electro-optic directional couplers having complementary asymmetry are used to reduce the wavelength sensitivity and relax manufacturing tolerance of 50/50 splitters in a Mach-Zehnder type switch or modulator. As taught by Thaniyavarn, asymmetry of the propagation constants of the directional couplers is achieved by voltage induced linear electro-optic effect, or by asymmetric waveguide widths. The former method allows the asymmetry to be tuned after fabrication of the coupler, whereas the latter method relies on tight control of fabrication processes to achieve a particular asymmetry.
Asymmetric directional couplers in electro-optic material are also demonstrated in a switch disclosed by Henning Bülow and Kurt Aretz in the Journal of Lightwave Technology, Vol. 7, No. 12, December 1989, entitled “Design and Realization of an Integrated Optic Switch for Crossbar Switching Arrays.” In this article the authors describe an electro-optic 2×2 switch including a bent bridge waveguide coupled through two directional couplers between crossed input and output waveguides. Asymmetry introduced by a tapered directional coupler and waveguide width mismatch blocks coupling via the directional couplers in the no voltage cross state. “The phase constant of waveguide 2 is slightly higher than the phase constant of waveguide 1. Due to this detuning, minimum power transfer occurs within the couplers and nearly no power appears at port p3.” Full switching voltage must be applied to both couplers to compensate for the asymmetry and permit light to pass into and out of the bent waveguide to drive the switch to the bar state. In this design the directional coupler asymmetry and taper geometry prevent unintentional coupling in the no voltage state. The transfer curve of input voltage versus coupling percentage has been shifted from a full coupling operation point at zero input volts to a zero coupling operation point at zero input volts. This switch design offers no change to the voltage required as compared to the prior art. Specifically, it is not suggested that an asymmetric design that shifts the operating point to partial coupling could be used to reduce switching voltage.
Directional couplers have advantages over other electro-optic devices since the close placement of the coupled waveguides permits an electric field to be effectively driven through both waveguides. This has been used to advantage in some prior art designs where two electrodes are positioned above the waveguides to be operated in push-pull operation and both waveguides are influenced by the same voltage equally but in opposite polarity. This configuration effectively reduces the required voltage by half. A push-pull directional coupler is disclosed in U.S. Pat. No. 4,820,009 by Suwat Thaniyavarn, issued to TRW Inc. on Apr. 11, 1989. This patent discloses a symmetrical directional coupler in which further reduction in voltage is achieved by including a passive Y-splitter in the structure to divide input light equally between the two waveguides in order to eliminate an electrical dc bias input. The Y-splitter is essential to this design. However, it is a very difficult structure to manufacture successfully. An error in the fabricated coupling length of the directional coupler will result in incomplete coupling that cannot be corrected by voltage. This means as discussed above, that full extinction would not be possible, and the dynamic range of the device would be limited.
Directional couplers are particularly interesting for the application as variable optical attenuators (VOA). In a VOA light input can be selectively attenuated to a desired output percentage, the remaining light being directed to the other output or into an attenuating medium.
One problem with prior art high-speed variable optical attenuators fabricated in lithium niobate is the large voltage that must be applied to the device. In typical operation, the attenuation remains constant for a long period of time. The high E-fields within the device resulting from the high voltages accelerate bias drift mechanisms, further increasing the maximum required voltage over the operational lifetime of the device. For example, if 20V is required to turn the VOA from minimum to maximum attenuation, and bias drift can cause a 2× increase in applied voltage in order to maintain the same attenuation, then the End-Of-Life (EOL) drive voltage can be as high as 40V, assuming the attenuation at Start-Of-Life (SOL) can be anywhere between the minimum and maximum value. If there is variability in the 2× bias drift multiplier, then even 40V of available drive voltage may be insufficient. Hence, reducing the SOL drive voltage to 10V or 5V greatly enhances the reliability of the VOA.
Bias drift mechanisms as discussed in U.S. Pat. Nos. 5,404,412 and 5,680,497 are due to mobile ions and space charge in the crystal and dielectric materials fabricated on top of the crystal.
An electro-optic directional coupler which overcomes the problems of the prior art, and which requires a reduced drive voltage remains highly desirable.